Functional analysis of the molecular factors controlling Qa1-mediated protection of target cells from NK lysis

CD94/NKG2 receptors on mouse NK cells recognize the nonclassical class I mo lecule Qa1 and can deliver inhibitory signals that prevent NK cells from ly sing Qa1-expressing cells. However, the exact circumstances under which Qa1 protects cells from NK lysis and, in particular, the role of the dominant Qa1-associated peptide, Qdm, are unclear. In this study, we examined in det ail the lysis of Qa1-expressing cells by fetal NK cells that express CD94/N KG2 receptors for Qa1 but that lack receptors for classical class I molecul es. Whereas mouse L cells and human C1R cells transfected with Qa1 were res istant to lysis by these effecters, Qa1-transfected TAP-deficient human T2 cells showed no resistance despite expressing high levels of surface Qa1. H owever, these cells could be efficiently protected by exposure to low conce ntrations of Qdm peptide or certain Qdm-related peptides. By contrast, even prolonged exposure of TAP-deficient RMA/S cells to high doses of Qdm pepti de failed to induce levels of surface Qa1 detectable with a Qa1-specific mA b or to protect them from NK lysis, although such treatment induced sensiti vity to lysis by Qa1-specific CTL. Collectively, these findings indicate th at high surface expression of Qa1 is necessary but not sufficient for prote ction, and that effective protection requires the expression of sufficient levels of suitable Qa1-peptide complexes to overcome activatory signals. Re sults obtained with a series of substituted Qdm peptides suggest that resid ues at positions 3, 4, 5, and 8 of the Qdm sequence, AMAPRTLLL, are importa nt for recognition of Qa1-Qdm complexes by inhibitory CD94/NKG2 receptors.